This invention relates to a method for the production of ribbons, suitable for use in filament winding, from a plastics material in film form and unidirectional fibrous (including filamentary) reinforcing materials, to the ribbons so obtained, and to the filament windings made therefrom.
The process of filament winding is commonly carried out using filaments or ribbons of unidirectional fibre which are impregnated with a solid or semi-solid, but still curable, resin composition. Impregnation of such fibres is usually carried out by immersing bundles of them in a tank of a liquid resin composition followed by evaporation of solvent and/or heating to solidify the resin composition. However, certain fibrous materials, such as carbon fibres, often cannot be satisfactorily impregnated in this way because the amount of resin composition which they pick up depends on the looseness of the bundle of fibres. Since this can vary between different parts of the same tow, pick-up of a controlled amount of liquid resin composition is impracticable. To overcome this difficulty, methods have been devised whereby impregnation is effected using a solid film of a curable resin composition which is applied to one or both sides of the fibres and the resin-fibre "sandwich" is heated under pressure so that the resin composition flows about the fibres to form a prepreg with controlled resin distribution. Such processes are described in British Patent Specification Nos. 1,299,177 and 1,257,482, and also in U.S. Pat. No. 3,660,197.
These methods are perfectly satisfactory when comparatively wide tape is required, but if a thin ribbon is required, such as from a single tow of fibres, the process is often too slow to be of practical use. Attempts have been made to overcome this difficulty by preparing a sheet of composite of normal width and slitting it into thin ribbons, as, for example, described in U.S. Pat. No. 3,700,511. Such a method, however, is not fully satisfactory since it is inevitable that some fibres on the edge of each ribbon are cut, and some materials, particularly aromatic polyamides, are very difficult to slit without causing the cut edges to fray. The advantages obtainable by the use of continuous lengths of reinforcement are thereby lessened.
There is therefore a need for a method of making ribbons of unidirectional fibre-reinforced plastics material in which the amount of plastics material in the ribbon is controlled but in which the ribbons have no cut edges.
We have found that this object may be achieved if a sheet of plastics material is used to impregnate discrete strips of unidirectional fibre having auxiliary strips of another material placed between all of the unidirectional fibre strips on a backing sheet. Removal of the auxiliary strips leaves impregnated unidirectional fibre ribbons on the backing.